This invention relates to a method of connecting electrical wires to connectors to produce highly reliable connections which are required when connecting components in data processing units, electrical appliances and automobile control units.
A conventional method of spot-welding a flat-type electrical wire 10, which includes a conductor 11 sandwiched between insulators 12 and 12', to a metal terminal 20 is illustrated in FIG. 1. One end portion of the flat-type electrical wire, after the conductor 11 has been exposed as illustrated, is inserted into the metal terminal 20 in such a manner that a projection 22 on a metal plate 23 of the metal terminal 20 is in contact with a portion of the insulator 12' opposite the exposed conductor. Thereafter, a preheating current, I.sub.p, is applied to the metal terminal through an electrode 30 of a spot welding machine, causing the metal terminal 20 to generate heat. The heat thus generated melts the portion of insulator 12' in contact with the projection 22. As a result, a welding current, I.sub.w, flows as shown in FIG. 2.
The projection 22 is brought into contact with the insulator 12' of the electrical wire 10 when the latter is inserted into the metal terminal 20 for the purpose of pushing away the insulator 12' made molten by the preheating current, I.sub.p, and thereby cause the projection 22 to contact the conductor 11.
In the conventional method described above, the projection extends from the metal plate 23 which is on the insulator 12' side of the electrical wire 10 when the electrical wire is inserted into the metal terminal 20. Therefore, the contact resistance between the conductor 11 and the projection 22 may fluctuate depending on the force imparted by the projection while pushing on the molten insulator. Furthermore, because of changes, even small changes, in the force imparted by the projection on the conductor 11, the contact resistance between the projection and the conductor may be larger or smaller than that between the conductor 11 and the metal plate 24 which has no projection. A higher contact resistance will produce a greater welding strength. Therefore, the conductor and the metal plate 23 having the projection 22 may be welded together more firmly than the conductor and the metal plate 24 having no projection, or vice versa. That is, the welding strength might not be uniform.